I. Field of the Invention
This invention relates generally to apparatus for accurately measuring the orientation of an unconstrained body with respect to a fixed reference system, and more particularly to a system used principally in military aircraft wherein the aiming point is designated by sighting through a viewing system incorporated into the pilot's helmet.
II. Description of the Prior Art
A principal use for the present invention is in target designation. When the pilot identifies a target to be attacked, he causes a reticle in his helmet visor system to coincide with the target and indicates that coincidence by means of a push-button switch control. At that instant, the orientation measurement system must be able to precisely identify the orientation of the helmet with respect to a fixed reference, typically the air frame.
It is very desirable that the orientation measurement system operates in a manner which in no way constrains the movement of the pilot's head. Two non-contact methods for performing this measurement are well known in the prior art, namely, optical and magnetic. Optical systems, such as that disclosed in the LaRussa U.S. Pat. No. 4,439,755 employ a collimated optical source fixed to the pilot's helmet and precisely oriented with respect to the line-of-sight axis. An optical receiver, which is sensitive to the angle of arrival of the optical beam transmitted from the helmet, is used to determine the orientation of the helmet. A commonly used method for determining the angle of arrival of the beam is to employ a dual-axis photo-detector in which X and Y axis analog voltages provide an electrical output which is proportional to the angle of arrival of the beam of light impinging on t he photo-detector.
Such optical systems, however, suffer a number of significant limitations. First of all, an unobstructed line of sight must be maintained between the optical transmitter and receiver for all possible orientations of the helmet. This requirement often may be difficult to achieve in modern cockpit configurations. While oftentimes all optical systems can maintain a high degree of accuracy over limited field-of-view, the accuracy is diminished over a practical field-of-view of .+-.70.degree. of azimuth or pitch and .+-.120.degree. of azimuth or yaw which is typically required.
U.S. Pat. No. 4,396,885 to Constant describes a magnetic system which is capable of an increased dynamic range of operation, which thereby translates into higher accuracy over a large field-of-view. In a typical magnetic system, such as that exemplified by the Constant patent, the helmet transmitter is comprised of three mutually orthogonal magnetic coils, one each for the X, Y and Z axis with respect to the line-of-sight reference of the helmet reticle. Likewise, the receiver is comprised of three mutually orthogonal coils, one each for the X, Y and Z axis with respect to the fixed reference of the air frame. An operational system, as will be subsequently described in more detail, relies on an accurate measurement, axis-by-axis, of the mutual magnetic inductive coupling between a transmitter coil and its respective corresponding receiver coil. As in the system of the above-referenced Constant patent, the measurement is generally accomplished using a single AC frequency for exciting the three transmitter coils in sequence and tuning the receiver to reject everything but that single frequency and, thus, generally rejecting all stray magnetic signals which might otherwise introduce error. The AC excitation frequency determines the tracking rate of the measurement system. An excitation frequency of the order of 10 KHz is required for a system to accurately track the rapid head motions that would normally be encountered. Unfortunately, at this range of frequency, the position accuracy of the system may be severely degraded due to eddy current errors. That is to say, at higher frequencies, the AC magnetic field generated by any of the transmitter coils may induce eddy currents in all of the surrounding conductive material, such as the aircraft fuselage. These eddy currents, in turn, generate an associated magnetic field. The net result is that the surrounding conductive material distorts the transmitted magnetic field, so as to introduce an error in the position measurement. There is, thus, no practical prior art system which combines the very desirable features of a wide field-of-view, accurate position determination, and rapid tracking of the helmet motion.
It is the principal object of this invention to perform the direction measurement in such a way that all of the above-mentioned desirable features are simultaneously and effectively realized.